This invention relates to an electric incandescant lamp provided with a tubular gas-tight glass bulb, of which at least one end has a seal. Current supply conductors extend through the wall of the bulb and carry a helical filament. A metallic envelope having a substantially rectangular cross-section and welding tongues accommodates a seal of the bulb. The envelope tongues are welded to a metallic centering base plate which is provided with a central hole and two contiguous parallel gaps, in which a respective welding tongue extends.
Such a lamp of the H3 type is known from French Patent No. 2,341,197 to which U.S. Pat. No. 4,122,367 corresponds. The lamp comprises a metallic envelope in which welding tongues are cut out taking the form of hooks and bent so as to project from walls of the envelope, these tongues being arranged in the gaps of the centering base plate and being fixed to lugs cut and folded from the base plate. Thus, the tongues of the envelope having a comparatively small thickness cover the comparatively thick lugs of the centering base plate to which the said tongues are welded.
In the case of lamps of the H3 type, one of the current supply conductors is connected to an insulated cable, the other one being connected to the metallic envelope.
Such a structure can be used only for a lamp of the H3 type. In fact, the standardization of the lamps of the H3 type does not impose maximum limit values on the distance between the welding tongues of the metallic envelope after the latter have been welded to the base plate. This permits of forming the lugs of the centering base plate in a sufficient thickness to withstand mechanical pressure during the spot welding of the tongues of the envelope.
For given lamp types, such as, for example, a lamp of the H1 type, a maximum limit value is imposed on the thickness of the metallic envelope of the lamp in a zone lying between the metallic centering base plate and the bulb, the said zone being adjacent to the base plate. Consequently, it is not possible to use the structure described in the aforementioned French Patent, which structure is only suitable in standardized lamps of the H3 type.
In order to satisfy the standardization requirements, the lamps of the H1 type which are commercially available have a metallic envelope traversing a base plate, of which four rectangularly bent cutout parts are spot-welded to the walls of the metallic envelope, the part of the envelope adjacent to the base plate and situated opposite to the part carrying the bulb having a conducting contact coated with an insulating resin and secured by means of a stamping tool. One of the current supply conductors is connected to this conducting contact, while the other one is connected to the metallic envelope.
However, these lamps have a disadvantage. In the electric incandescent lamps used in optical systems, the position of the filament with respect to the optical system is subjected to narrow tolerances. With the known H1 lamps, it is difficult to observe these tolerances due to the fact that the metallic base plate is held in place without a support with respect to the metallic envelope at the moment of spotwelding of the four cut out parts which are fairly thick and rigid.
Moreover, the insulating material coating the central contact is often a ceramic material, which insulates better, but is more liable to break. This material cannot be used in a known design of H1 type lamps because, if ceramic material is secured in the lower part of the envelope by means of a stamping tool, this leads to the destruction of this material.